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The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal.It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The internal anal sphincter forms the walls of the anal canal. The internal anal sphincter is not under voluntary control, and in normal persons it is contracted at all times except when there is a need to defecate. This means that the internal anal sphincter contributes more to the resting tone of the anal canal than the external anal sphincter.
The sphincter urethrae, or urethral sphincter, controlling the exit of urine from the body. At the anus, there are two anal sphincters which control the exit of feces from the body, the internal anal sphincter and external anal sphincter. The inner sphincter is involuntary and the outer is voluntary.
Females do have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibers wrap around the vagina and urethra and contraction leads to constriction of both the vagina and the urethra.
Contractions of abdominal and pelvic floor muscles can create intra-abdominal pressure, which further increases intra-rectal pressure. The internal anal sphincter (an involuntary muscle) responds to the pressure by relaxing, thus allowing the feces to enter the canal.
The rectoanal inhibitory reflex (RAIR), also known as the anal sampling mechanism, anal sampling reflex, rectosphincteric reflex, or anorectal sampling reflex, is a reflex characterized by a transient involuntary relaxation of the internal anal sphincter in response to distention of the rectum. [1]
[15] [16] Subconscious processes are controlled by the autonomic nervous system; these are involuntary movements such as contraction of the smooth muscle of the internal anal sphincter or the colon. The autonomic nervous system also provides sensory information; this could be about the level of distension within the colon or rectum.
The internal urethral sphincter is a urethral sphincter muscle which constricts the internal urethral orifice. It is located at the junction of the urethra with the urinary bladder and is continuous with the detrusor muscle , [ 1 ] [ 2 ] but anatomically and functionally fully independent from it. [ 3 ]